1. Technical Field
The present invention relates generally to a spark plug of the type used in an electric spark ignition system of an internal combustion engine, and, more particularly, to a variable gap spark plug.
2. Discussion of the Related Art
Spark ignition of an internal combustion engine generally involves igniting a mixture of air and fuel with an electric discharge (xe2x80x9csparkxe2x80x9d). FIG. 1 illustrates a conventional spark plug 10 having a housing 12 configured to retain an insulator body 14. Plug 10 includes a central terminal 16 comprising a central electrode 18, and a ground terminal 20 comprising a ground electrode 22. The space between an end surface 24 of central electrode 18 and ground electrode 22 defines a gap 26. The spark occurs across gap 26. The distance of gap 26 of plug 10 is fixed. One shortcoming of a fixed gap distance is that it is a compromise between startability of a cold engine, which improves as the distance is reduced (up to some predetermined minimum gap), and a dilution tolerance (air or EGR) of a warm engine, which improves as the distance is increased due to a higher voltage elongated spark. Accordingly, neither startability nor dilution tolerance is optimized using a conventional, fixed gap spark plug.
It is known, however, to provide a spark plug having dual gaps, as seen by reference to U.S. Pat. No. 4,514,657 issued to Igashira et al. entitled xe2x80x9cSPARK PLUG HAVING DUAL GAPS FOR INTERNAL COMBUSTION ENGINES.xe2x80x9d Igashira et al. disclose a central electrode, an earth electrode and a supplementary electrode provided on the earth electrode or the tip end of the central electrode. The supplementary electrode cooperates with one of the other electrodes to form a spark gap which is smaller in dimension than the normal spark gap defined between the earth and central electrodes. Igashira et al. disclose that the small gap is used at the beginning of the discharge to enable a reduction in the discharge voltage demand. Neither gap is disclosed as being selected based on an operating temperature. This is not surprising since, at cold temperatures, the larger gap remains active, and at warmer engine operating temperatures, the reduced dimension gap remains active. It is believed that maintaining both sized spark gaps through an operating temperature range is less than optimally efficient, in terms of energy usage.
There is therefore a need to provide a spark plug that minimizes or eliminates one or more of the shortcomings as set forth above.
An advantage of the present invention is that it optimizes cold startability of an internal combustion engine by providing a first, smaller spark gap, while also optimizing warm engine operation by expanding to a larger spark gap in response to an increase in temperature.
In one embodiment according to the invention, a spark plug is provided for igniting a fuel mixture within a combustion chamber of an internal combustion engine. The plug includes a housing formed of electrically-conductive material, a central electrode having an end surface, and a ground electrode coupled to the housing. The ground electrode is disposed in a first position, at a first temperature, opposing the end surface to form a spark discharge gap therebetween, characterized in that: the ground electrode is configured to deflect away from the end surface of the central electrode to a second position in response to an increase in temperature to a second temperature. The deflection increases the spark discharge gap.
Other objects, features, and advantages of the present invention will become apparent to one skilled in the art from the following detailed description and accompanying drawings illustrating features of this invention by way of example, but not by way of limitation.